The term “speckle” refers to the random distribution of light reflected off a surface. Speckle typically has a high contrast field of spots (or “speckles”) that are randomly distributed in position but relatively uniform in their size and brightness. The individual speckles in a speckle observation plane have an average size that is directly proportional to the wavelength and inversely proportional to the viewing angle subtended at the observation plane by the illuminated area of the surface. Thus, when the position of the detector plane is fixed with respect to the observation plane, the size of the speckles can be controlled by the size of the illumination area.
Increasing or decreasing the distance between the diverging (or converging) light source and the surface is one technique that alters the size of the illumination area. Another technique is to place a lens with an appropriate focal length between the light source and the surface. In FIG. 1, which is a diagrammatic illustration of an optical illumination system according to the prior art, lens 102 is placed between coherent light source 104 and surface 106. When the beam of light 108 is emitted towards surface 106, lens 102 directs the beam towards illumination area 110. Light 112 reflecting off surface 106 creates speckle.
Including lens 102 in system 100 increases the cost and complexity of system 100. Lens 102 must be positioned and centered at the proper distance from light source 104 to effectively direct the light towards illumination area 110. And lens 102 cannot efficiently and significantly transform the intensity profile of illumination area 110 despite being able to change the size of illumination area 110.
FIG. 2 depicts an intensity profile for illumination area 110 in FIG. 1. The intensity profile of FIG. 2 is typically generated when light 108 underfills lens 102. The intensity profile includes regions 200, 202, 204 which have varying intensities. Region 200 illustrates an area where the intensity of the beam is greatest. Region 202 depicts an area with less intensity compared to region 200 while region 204 illustrates an area having less intensity than region 202. This non-uniform intensity profile is known as a Gaussian profile and is typically undesirable for creating and detecting speckle patterns for motion measurements and speckle navigation.
With a Gaussian profile, an optical element such as aperture 116 (see FIG. 1) must be positioned such that a compromise is reached between the optical efficiency of the system and the uniform illumination of the viewable area. A small aperture (α<<Θ) causes illumination area 110 to appear more uniform when viewed from detector 114 (when 114 is aligned to be centered on the brightest portion of the intensity profile, region 200). But when the aperture 116 is small, a considerable amount of light does not strike detector 114. A larger aperture 116 transmits more light to detector 114 but the portion of illumination area 110 viewed by detector 114 has poor uniformity.